Removed the distinction between generic Ltac vars and Let/Intro bindings, which permits using only one environment for interning terms.

Ltac semantics was sligthly changed, as it required introducing
a lot of additional coercions from goal variables to other types.
Ltac seemed to be quite non-uniform, as it tried to represent
hypotheses with intropatterns, instead of the dedicated var type.

git-svn-id: https://scm.gforge.inria.fr/svn/coq/trunk@16612 85f007b7-540e-0410-9357-904b9bb8a0f7